Have done a few of these in my time. Not easy, but can be done. I have taken 2 rows of shingles off, difficult because you have to get nails from not only the course you want to remove, but also, the row above that. Once the row is removed, I cut a 3 inch square out of the top of the course that is showing and keep that to be replaced. Drill your holes, fill with cellulose, install a tapered wooden plug, cover the plug with roof tar, replace the square you cut out earlier, cover this with a thin coat of roof tar, reinstall your shingle. Labor intensive, but I have had great success with it. Good luck.

It seems that you describe filing the entire cavity between the vertical interior wall and the steeply pitched lower roof with insulating material - is this correct? How much insulation is this? How do you ensure that you have sealed the open joist cavity ends? In some of these built in recent times, we have seen the plywood interior subfloor extend out to the steeply angled roof so that filling the cavity with insulation does not address the open cavities - and these are the biggest problem with this type of mansard. Second biggest problem in this particular case is the open party wall at both ends of the house (In this case, just one end because it is an end-row unit). This open party wall would need some dense-packing. Both the party wall and the joist cavities are the source of all the moisture that might be in the lower attic - in this DC climate.

You could then drill a 1 inch hole thru the soffit and fill the open end of the floor joists. If the soffit is large enough, drill your normal sized hole and fill it, plug, and patch. You can expect to go thru alot of material as the lower portion flairs out. Anything can be done, only limitations are your comfort levels.

Yikes. It is never as straightforward as one would hope in these "do it right" retrofits. The homeowner is getting itchy about implementing the work, especially because he has been given much misinformation (so it appears) from other "blow it and roll" contractors.

I may have to walk away from this one if he feels confident enough to just settle with more insulation is better than no insulation.

You put your finger on it: air seal floor cavity! The volume is triangular and the weight and force with dense pack will be borne by the soffit. I love cellulose, but may be best to open it up from below (or if re-roofing pull back the sheathing), do some air sealing and ensure your duct runs are"inside" the house.